Gas turbine engines, in particular so-called turbofan engines, are commonly used to provide propulsion for a wide range of modern aircraft. Such engines typically include a bypass duct through which a proportion of the air pressurized by the fan is passed and a fan nozzle for producing thrust from the fan-pressurized bypass air. The remaining air is passed through the engine core in which it is used as the working fluid to generate power for the fan.
Such engines are typically supported within a nacelle that is secured to the structure of the aircraft, for example to the fuselage or to the underside of the wing, by means of a pylon. The nacelle typically comprises an outer cowl, defining the external housing of the engine and within which the fan is disposed, and an inner cowl which houses the core of the engine, i.e. the turbine and combustion chamber stages of the engine. The inner and outer cowls are generally cylindrical in section and are aligned substantially concentrically and generally parallel with the main or thrust axis of the engine. The bypass duct is defined by the generally annular space between the radially inner and outer cowls and includes a fan nozzle at its exit.
In many turbofan engines, the outer cowl includes a thrust-reverser section located towards the rear of the nacelle. In such arrangements, the rear section of the outer cowl is moveable, for example translatable, relative to the forward section of the outer cowl so as to enable the deployment of blocking devices which cause the pressurised air from the fan to be diverted forwardly and impart a retardation force on the aircraft during braking.
In order to facilitate access to the engine by maintenance personnel, in some turbofan engines the thrust-reverser section of the cowl is divided into two halves known as C-ducts each of which is hinged to the pylon at its upper edge for rotation about a thrust-reverser hinge-line extending generally parallel to the main axis of the engine. Rotation of these C-ducts (which effectively constitute a pair of clamshell-type outer cowl doors and are therefore hereafter termed thrust-reverser cowl doors) about the thrust-reverser hinge-line affords access to the components of the engine by maintenance operators.
In such engines, it is common for the section of the inner cowl corresponding to the thrust-reverser section also to be divided into two halves (hereafter termed core cowl doors) with each half being fixed to, or integrally formed with, the corresponding thrust-reverser cowl door. The space between the core cowl door and the corresponding thrust-reverser cowl door defines a portion of the bypass duct as described above.
In use, during inspection or maintenance of the engine, the thrust-reverser cowl doors are opened by rotation either manually or hydraulically by means of a power door opening system (PDOS). Rotation of the thrust-reverser cowl doors about the thrust-reverser hinge-line causes corresponding rotation of the core cowl doors, which are connected thereto, so as to provide access to the engine core components.
However, the applicant has recognised that, in cases where the engine is suspended by a pylon below the wing of an aircraft such that the thrust-reverser section is disposed beneath the leading edge of the wing, the amount of rotation, i.e. the degree of opening, of the thrust-reverser cowl doors is generally limited by their clearance to the lower surface of the wing. Since the inner cowl doors are fixed to the thrust-reverser cowl doors and are therefore not independently moveable relative thereto, the degree of opening of the inner cowl doors is similarly limited. Thus, overall accessibility of the engine core for maintenance purposes is restricted.
Modifying the shape or configuration of the nacelle or wing in order to improve clearance between the thrust-reverser cowl doors and the underside of the wing may result in sub-optimal aerodynamic characteristics. On the other hand, lengthening the pylon in order to increase the distance between the engine and the underside of the wing positions the engine closer to the ground, increasing the risk of damage to the engine by ingestion of debris. Finally, complete removal of the cowl doors may improve access to the engine core, but significantly increases maintenance operator time and costs.
There is therefore a need to improve the degree of access to the engine core when the inner and outer cowl doors are opened whilst obviating the inherent disadvantages of the above-mentioned solutions. It is an aim of the present invention to address this problem. Embodiments of the invention may improve maintenance access to the core of a turbofan engine by providing a nacelle for an engine comprising independently rotatable inner and outer cowl doors which can be selectively connected to rotate about different hinge lines. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.